Serialize the maigret graph (the same one --graph builds) into an idempotent Cypher script importable into Neo4j. Reuses MaigretGraph via an extracted _build_maigret_graph() helper, leaving save_graph_report behavior unchanged; no new runtime dependency. Adds the --neo4j flag, the neo4j_report setting, docs, and a unit test.
Closes#2630
